Form Regulation in Harenactis attenuata 301 
same length as the others these two groups would appear, except 
for the smaller number of tentacles, like complete discs. Mouth 
and cesophagus are, however, absent and never appear. Fig. 13 
shows a section through the group a and group 4 is quite similar. 
Aboral tentacles never appear in the groups c and d, but the three 
tentacles present form part of a circle. Fig. 14shows the position 
Fics. 10-14 
of the middle tentacle of the groups c and d with reference to the 
region of union. ‘The two lateral tentacles of each of these groups 
arise nearer the line of union than does the middle tentacle and 
may perhaps be oral-aboral, though as will appear below it seems 
probable that in many cases the terms oral and aboral retain 
only a regional significance in the region of union. 
